A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate, and critical linewidth (critical dimension commonly abbreviated to CD). Overlay and CD can be measured in photosensitive resist (after exposure and before or after development), or in actual product features formed by etching, deposition and the like. These measurements may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making direct measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties.
Compared with electron microscopy, scatterometry is enabling relatively rapid measurement of CD and overlay to be performed in an instrument which is closely integrated within the lithographic production cell or cluster. The results of these measurements can be fed back, or fed forward into control systems of the lithographic apparatus or other processing tools, so as to adjust performance more interactively. Scatterometry nevertheless generally makes use of a complex and computationally demanding instrument that is provided next to the lithographic apparatus itself. The lithographic apparatus, though it contains very accurate metrology systems for positioning and mapping the substrate and patterning device to place all parts of the applied pattern at their desired positions, usually does not directly measure overlay or CD.
Depending on the application, control of overlay and CD may be critical to good performance of the manufactured device. In the highest density structures, made today by double patterning techniques, it can be important not only to keep CD within a certain range, but also to match the CD achieved in different process steps.